1. Field of the Invention
The present invention relates to a partially fluorinated copolymer based on a trifluorostyrene and a substituted vinyl compound, and to an ionic conductive polymer membrane formed of the same. More particularly, the present invention relates to a partially fluorinated copolymer with trifluorostyrene units and substituted vinyl compound units, and to an ionic conductive polymer membrane, formed of the copolymer, which has excellent mechanical properties and a low degree of swelling caused by water absorption, and to a fuel cell having the ionic conductive polymer membrane.
2. Description of the Related Art
Recently, with the advance of portable electronic devices and wireless communications devices, the need for high performance fuel cells for these portable devices has increased. In order to improve the efficiency of fuel cells, a polymer membrane which ensures a high ionic conductivity and reduces the cross-over of fuel, particularly of methanol, is needed.
The fuel cell is a power generating system which converts the energy generated by electrochemical reaction of a fuel and an oxidizing gas to electrical energy for use. Fuel cells are classified into, for example, fuel cells with a molten carbonate salt electrolyte operable at a high temperature of 500–700° C., fuel cells with a phosphoric acid electrolyte operable around 200° C., fuel cells with an alkali electrolyte operable in a wide range of temperature from room temperature to 100° C. or less, fuel cells with a proton exchange membrane as electrolyte, and fuel cells with a solid electrolyte operable at a high temperature of 600–1000° C.
Polymer electrolyte fuel cells include the proton exchange membrane fuel cell (PEMFC) using hydrogen gas as fuel, and the direct methanol fuel cell (DMFC) which uses liquid methanol from the anode as direct fuel.
The proton exchange membrane fuel cell (PEMFC), a future clean energy source emerging as a substitute for fossil energy, has high output density and energy conversion efficiency. Also, the PEMFC is workable at room temperature and is easy to seal and miniaturize, so that it can be extensively used in the fields of pollution-free vehicles, household power generating systems, mobile telecommunications, portable power generating systems, and medical, military and space equipment.
The PEMFC is a power generator for producing direct current through the electrochemical reaction of hydrogen and oxygen. The basic structure of such a cell is shown in FIG. 1. Referring to FIG. 1, the PEMFC has a proton exchange membrane 11 interposed between the anode and the cathode.
The proton exchange membrane 11 is formed of a solid polymer electrolyte with a thickness of 20–200 μm. The anode and cathode include support layers 14 and 15 for supplying reaction gas, and catalytic layers 12 and 13 in which oxidation/reduction of the reaction gas occurs, which collectively form a “gas diffusion electrode.” In FIG. 1, reference numeral 16 represents a current collector.
In the PEMFC having the above structure, with the application of hydrogen gas as a reaction gas, hydrogen molecules are decomposed into hydrogen ions and electrons by an oxidation reaction in the anode. The hydrogen ions so produced reach the cathode through the proton exchange membrane 11.
Meanwhile, in the cathode oxygen molecules take electrons from the anode through the membrane and are reduced to oxygen ions by reduction. The oxygen ions so produced react with hydrogen ions from the anode and produce water molecules.
As shown in FIG. 1, in the PEMFC, the catalytic layers 12 and 13 of the gas diffusion electrode are formed over the support layers 14 and 15, respectively. The support layers 14 and 15 are formed of carbon cloth or carbon paper. The surface of the support layers 14 and 15 are treated so as to allow easy passing of reaction gas, of water to the proton ion exchange membrane 11, and of water obtained as the reaction product.
On the other hand, the DMFC has the same structure as the PEMFC, but uses liquid methanol, instead of hydrogen, as a reaction fuel. As the liquid methanol is supplied to the anode, an oxidation reaction occurs in the presence of a catalyst, so that hydrogen ions, electrons and carbon dioxide are generated. The DMFC has poor cell efficiency compared with the PEMFC because of lower catalytic activities of the anodic fuels. However, use of liquid fuel makes its application to portable electronic devices easier.
The previously mentioned fuel cells usually employ an ionic conductive polymer membrane as the proton exchange membrane disposed between the anode and the cathode. The ionic conductive polymer membrane, as an electrolyte of the fuel cell, serves to transfer hydrogen ions from the anode to the cathode, and prevents the fuels for each of the anode and cathode from being mixing with each other. In addition, the ionic conductive polymer membrane is formed of a polymer membrane with sulfonyl groups as side chains. Because the polymer membrane contains water, a sulfonic acid group of the polymer electrolyte is dissociated in the water medium so that the sulfonyl group is produced with ionic conductivity.
The ionic conductive polymer membrane should have the following characteristics: a high ionic conductivity, electrochemical stability, mechanical properties suitable as a membrane, thermal stability at working temperature, easy processing into a thin film for reduced resistance, low cost and low degree of swelling caused by liquid absorption, etc.
The most widely known ionic conductive polymer membrane has a polytetrafluoroethylene backbone having sulfonyl group as side chains. However, this polymer membrane is of little practical use due to its high manufacturing cost and complicated manufacturing process. To solve these problems, an ionic conductive polymer membrane formed of polytrifluorostyrene, a partially fluorinated polymer, has been suggested in U.S. Pat. Nos. 5,422,411; 5,498,639; 5,602,185; 5,684,192; 5,773,480; and 5,834,523. However, the ionic conductive polymer based on sulfonated polytrifluorostyrene is known to be very brittle and thus has difficulty in practical use. For this reason, to enhance the mechanical strength of the polymer membrane, there has been used a copolymer of trifluorostyrene, and a polymer which is able to be polymerized with triflurostyrene, for example, a monomer containing fluorine with a long side chain, such as heptadecafluorodecyl acrylate, heptadecafluorodecene, hydroxypropyl methacrylate, hydroxybutyl acrylate and hydroxyethyl methacrylate.
When the ionic conductive polymer membrane is applied to a fuel cell, the polymer membrane absorbs water and serves as a medium which allows hydrogen ions to pass. As the polymer membrane absorbs water, the thickness and the area of the polymer membrane varies by swelling. However, if the degree of the swelling is excessive, due to the difference between the dry state and the full water absorption state, manufacturing a fuel cell with the polymer membrane becomes difficult.